Negative electrode including silicon nanoparticles having a carbon coating thereon

ABSTRACT

An example of a negative electrode includes silicon nanoparticles having a carbon coating thereon. The carbon coating has an oxygen-free structure including pentagon rings. The negative electrode with the silicon nanoparticles having the carbon coating thereon may be incorporated into a lithium-based battery. In an example of a method, silicon nanoparticles are provided. A carbon precursor is applied on the silicon nanoparticles. The carbon precursor is an oxygen-free, fluorene-based polymer. Then the silicon nanoparticles are heated in an inert gas atmosphere to form the carbon coating on the silicon nanoparticles. The carbon coating formed on the silicon nanoparticles has an oxygen-free structure including pentagon rings.

INTRODUCTION

Secondary, or rechargeable, lithium-based batteries are often used inmany stationary and portable devices, such as those encountered in theconsumer electronic, automobile, and aerospace industries. The lithiumclass of batteries has gained popularity for various reasons, includinga relatively high energy density, a general nonappearance of any memoryeffect when compared to other kinds of rechargeable batteries, arelatively low internal resistance, and a low self-discharge rate whennot in use. The ability of lithium batteries to undergo repeated powercycling over their useful lifetimes makes them an attractive anddependable power source.

SUMMARY

An example of a negative electrode includes silicon nanoparticles havinga carbon coating thereon. The carbon coating has an oxygen-freestructure including pentagon rings. The negative electrode, with thesilicon nanoparticles having the carbon coating thereon, may beincorporated into a lithium-based battery. The lithium-based batteryalso includes a positive electrode and a microporous polymer separatorsoaked in an electrolyte solution. The microporous polymer separator isdisposed between the positive electrode and the negative electrode.

In an example of a method, silicon nanoparticles are provided. A carbonprecursor is applied on the silicon nanoparticles. The carbon precursoris an oxygen-free, fluorene-based polymer. Then the siliconnanoparticles are heated in an inert gas atmosphere to form a carboncoating on the silicon nanoparticles. The carbon coating formed on thesilicon nanoparticles has an oxygen-free structure including pentagonrings.

DRAWINGS

Features of examples of the present disclosure will become apparent byreference to the following detailed description and drawings, in whichlike reference numerals correspond to similar, though perhaps notidentical, components. For the sake of brevity, reference numerals orfeatures having a previously described function may or may not bedescribed in connection with other drawings in which they appear.

FIG. 1 is a schematic view of an example of a silicon nanoparticlehaving a carbon coating thereon;

FIG. 2 is a schematic, cross-sectional view of an example of a negativeelectrode, including an example of the carbon coated siliconnanoparticles disclosed herein, on a current collector;

FIG. 3 is a schematic, cross-sectional view of another example of anegative electrode, including an example of the carbon coated siliconnanoparticles disclosed herein, on a current collector;

FIG. 4 is a schematic, cross-sectional view of an example of a lithiumsulfur battery that has a negative electrode including an example of thecarbon coated silicon nanoparticles disclosed herein;

FIG. 5 is a cross-sectional, schematic view of an example of a lithiumion battery that has a negative electrode including an example of thecarbon coated silicon nanoparticles disclosed herein; and

FIG. 6 is a graph illustrating the capacity retention (Y axis) versusthe cycle number (X axis) of three comparative batteries and an examplebattery with a negative electrode including an example of the carboncoated silicon nanoparticles disclosed herein.

DETAILED DESCRIPTION

Lithium-based batteries generally operate by reversibly passing lithiumions between a negative electrode (sometimes called an anode) and apositive electrode (sometimes called a cathode). The negative andpositive electrodes are situated on opposite sides of a porous polymerseparator soaked with an electrolyte solution that is suitable forconducting the lithium ions. During charging, lithium ions are inserted(e.g., intercalated, alloyed, etc.) into the negative electrode, andduring discharging, lithium ions are extracted from the negativeelectrode. Each of the electrodes is also associated with respectivecurrent collectors, which are connected by an interruptible externalcircuit that allows an electric current to pass between the negative andpositive electrodes. Examples of lithium-based batteries include alithium sulfur battery (i.e., includes a sulfur based positive electrodepaired with a lithiated negative electrode) and a lithium ion battery(i.e., includes a lithium-based positive electrode paired with anegative electrode or a non-lithium positive electrode paired with alithiated negative electrode).

Silicon nanoparticles may be used as the active material for a negativeelectrode. Fully lithiated silicon (Li_(4.4)Si) has a high gravimetriccapacity of about 2010 mAh and a high volumetric capacity of about 2400mAh, e.g., as compared to lithiated graphite (LiC₆), which has agravimetric capacity of about 340 mAh and a volumetric capacity of about712 mAh. However, silicon has a high volume expansion ofV_(Si):V_(Li4.4Si)=1:4.0, which means approximately 300% volumeexpansion (e.g., as compared to silicon suboxide, which may have avolume expansion of V_(SiOx):V_((Li4Si+Li4SiO4))=1:2.3, which meansapproximately 130% volume expansion). The high volume expansion ofsilicon may result in electrode fracture and loss of electrical contactand electrode integrity. Silicon also has a low electrical conductivityof about 1.5×10⁻³ S/m, e.g., as compared to carbon, which may haveelectrical conductivity greater than 10³ S/m. Additionally, a siliconelectrode may have a poor cycle life and active surfacesolid-electrolyte interphase (SEI) formation, which may cause continuouselectrolyte consumption and lithium loss.

Silicon nanoparticles may be coated with carbon to mitigate the highvolume expansion, low electrical conductivity, poor cycle life, andactive surface SEI formation of silicon electrodes. If the carboncoating has oxygen rich surface functional groups, these functionalgroups may consume active lithium and electrolyte, which may result inlow efficiency and cycle life. A carbon coating with oxygen rich surfacefunctional groups may be formed when oxygen rich carbon precursors areused, such as resorcinol-formaldehyde or glucose. Oxygen can be removedfrom the carbon coating by heating at a high temperature (e.g., greaterthan 1,000° C.). Additionally, the carbon coating precursors may beheated to increase the electrical conductivity of the carbon coating.However, heating at temperatures higher than about 850° C. may result inthe formation of a silicon carbide (SiC) layer between the carboncoating that is formed and the silicon nanoparticles on which the carboncoating is formed. Silicon carbide is an insulator of both electrons andlithium ions, and thus, the formation of a silicon carbide layer maydeleteriously affect the electrochemical performance of the carboncoated silicon nanoparticles.

In the negative electrode 24, 24′ (see FIGS. 2 and 3 ) disclosed herein,coated nanoparticles 10 are included as an active material. The coatednanoparticles 10 are made up of silicon nanoparticles 12 having a carboncoating 14 thereon. FIG. 1 schematically illustrates one coatednanoparticle 10, including one silicon nanoparticle 12 with the carboncoating 14 thereon. The carbon coating 14 has an oxygen-free structureincluding pentagon rings 16.

The method disclosed herein uses an oxygen-free, fluorene-based polymeras a carbon precursor and heats the silicon nanoparticles 12 (with thecarbon precursor applied thereon) in an inert gas atmosphere. The methodforms the oxygen-free carbon coating 14 on the silicon nanoparticles 12without having to heat the coating 14 to a temperature greater than 850°C. Thus, a silicon carbide layer is not formed in the coatednanoparticles 10.

Additionally, the oxygen-free, fluorene-based polymer is able to developelectrical conductivity at a relatively low temperature. In an exampleof the method, the silicon nanoparticles 12, with the carbon precursorapplied thereon, are heated at a temperature ranging from about 650° C.to about 750° C.

Further, the oxygen-free, fluorene-based polymer contains pentagonrings, which causes the carbon coating 14 to include pentagon rings 16.The presence of the pentagon rings 16 in the carbon coating 14 may giverise to curvatures 20 in the carbon coating 14, which effectively resistthe volume expansion of silicon. In some examples, the oxygen-free,fluorene-based polymer includes an allyl group, which may cross-link thepolymer and render a strong electron conducting network throughout thecarbon coating 14.

The method for forming the coated nanoparticles 10 includes providingthe silicon nanoparticles 12. The silicon nanoparticles 12 provided mayhave a particle size ranging from about 30 nm to about 100 nm.

The method also includes applying the carbon precursor on the siliconnanoparticles 12. Applying the carbon precursor may be accomplished bymixing the carbon precursor, the silicon nanoparticles 12, and a solventto form a slurry, and drying the slurry. In an example, the carbonprecursor, the silicon nanoparticles 12, and the solvent are mixed by aplanetary centrifugal mixer, e.g., a THINKY® Mixer for about 20 minutes.In another example, the slurry is dried in a hood over night (e.g., 12hours), followed by drying the mixture at 60° C. under vacuum for about12 hours.

The silicon nanoparticles 12 may be present in the slurry in an amountranging from about 10 wt % to about 50 wt % (based on the total wt % ofthe slurry). In an example, the silicon nanoparticles 12 make up about31.25 wt % of the slurry.

The carbon precursor may be present in the slurry in an amount rangingfrom about 1 wt % to about 10 wt % (based on the total wt % of theslurry). In an example, the carbon precursor is about 6.25 wt % of theslurry.

In an example, the weight ratio of the carbon precursor to siliconnanoparticles 12 in the slurry is 1:5.

As mentioned above, the carbon precursor is an oxygen-free,fluorene-based polymer. Examples of suitable oxygen-free, fluorene-basedpolymers include polymers formed (e.g., via a condensation reaction)from the following monomer(s): (i) 9,9-dioctylfluorene-2,7-diboronicacid bis(1,3-propanediol) ester

and (ii) 2,7-dibromofluorene

or a modified 2,7-dibromofluorene monomer (e.g., having an allyl groupattached to the 9,9′ position). Other examples of monomer (i) include9,9-Dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester

9,9-Didodecylfluorene-2,7-diboronic acid

or 9,9-Dihexylfluorene-2,7-diboronic acid

and other examples of monomer (ii) include2,7-Dibromo-9,9-dimethyl-9H-fluorene

2,7-Dibromo-9,9-diallyl-9H-fluorene

9,9-Didodecyl-2,7-dibromofluorene

and 9,9-Dihexyl-2,7-dibromofluorene

Any of monomer (i) (or another like monomer) may be reacted with any ofmonomer (ii) to form the oxygen-free, fluorene-based polymers disclosedherein. As another example, 9,9-Didodecylfluorene-2,7-diboronic acid maybe reacted with 2,7-Dibromo-9,9-diallyl-9H-fluorene.

While not being bound to any theory, it is believed that use of theoxygen-free, fluorene-based polymer as the carbon precursor allows thestructure of the carbon coating 14 to be oxygen-free without beingexposed to heating at high temperatures (e.g., greater than 1000° C.).Thus, the use of the oxygen-free, fluorene-based polymer contributes tothe ability of the carbon coating 14 to mitigate the high volumeexpansion, low electrical conductivity, poor cycle life, and activesurface SEI formation of silicon electrodes, without consuming activelithium and electrolyte or forming an electrically insulating siliconcarbide layer.

As also mentioned above, in some examples, the oxygen-free,fluorene-based polymer includes an allyl group. As an example, the twoprotons on the five-membered ring structure (9,9′ position) of2,7-dibromofluorene are acidic and can be modified with allyl groups.When they are present, allyl groups in the oxygen-free, fluorene-basedpolymer may crosslink the polymer. The crosslinked polymer may form astrong electron conducting network in the carbon coating 14. Thecrosslinked polymer may also be resistant to softening during the heattreatment (which will be discussed below). If the polymer does soften ormelt, the carbon coating 14 formed may be insufficiently porous and havea high electrical resistance.

The solvent may be present in the slurry in an amount ranging from about50 wt % to about 80 wt % (based on the total wt % of the slurry). In anexample, the solvent makes up about 62.5 wt % of the slurry. An exampleof the solvent includes chlorobenzene, dichlorobenzene, etc.

In one specific example, the carbon precursor is applied on the siliconnanoparticles 12 by mixing 1 g of the carbon precursor, 5 g of thesilicon nanoparticles 12, and 10 g of chlorobenzene with a THINKY® Mixerto form a slurry. In this example, the slurry may then be dried in ahood.

In some examples, the method may include synthesizing the carbonprecursor prior to applying the carbon precursor on the siliconnanoparticles 12. In an example, the carbon precursor may be synthesizedby stirring a mixture including fluorene-based monomers (i) and (ii), acatalyst, and a solvent. In an example, the mixture may be stirred in aninert (e.g., argon) gas atmosphere. In another example, the mixture maybe vigorously stirred for about 72 hours at 70° C. in tetrahydrofuran(THF).

The fluorene-based monomer(s) may be present in the mixture in an amountranging from about 2 wt % to about 50 wt % (based on the total wt % ofthe mixture). In an example, the fluorene-based monomer makes up about30 wt % of the mixture. As mentioned above, an example of thefluorene-based monomer includes 2,7-dibromofluorene (or a modifiedversion thereof), used in combination with9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester at a1:1 molar ratio. It is to be understood that when an oxygen-containing,fluorene-based monomer is used to synthesize the oxygen-free,fluorene-based polymer, the oxygen will become part of a removablebyproduct and will not be present in the synthesized polymer.

While some examples of the fluorene-based monomers are disclosed herein,it is to be understood that any fluorene-based monomers may be usedthat, when reacted via a condensation reaction, will form an oxygen-freefluorene-based polymer.

The catalyst may be present in the mixture in an amount ranging fromabout 0.01 wt % to about 1 wt % (based on the total wt % of themixture). In an example, the catalyst makes up about 0.035 wt % of themixture. An example of the catalyst includestetrakis(triphenylphosphine)palladium(0) ((PPh₃)₄Pd(0)). Other palladiumcatalysts may also be used.

The solvent may be present in the mixture in an amount ranging fromabout 30 wt % to about 95 wt % (based on the total wt % of the mixture).In an example, the solvent makes up about 80 wt % of the mixture.Examples of the solvent include tetrahydrofuran (THF), toluene, dimethylether (DME), diethyl ether (DEE), and the like.

After the carbon precursor is synthesized, it is to be understood thatthe mixture has been altered, and at least includes the oxygen-free,fluorene-based polymer, which is the carbon precursor. At least somesolvent may also be present in the altered mixture.

After the carbon precursor is synthesized, the carbon precursor may thenbe removed from the mixture using any suitable separation technique. Forexample, the carbon precursor may be removed by vacuum filtration,centrifugal force, or any other suitable means. The carbon precursor maybe washed multiple times with deionized water during and/or after theseparation of the carbon precursor from the altered mixture. It may bedesirable to wash the carbon precursor with deionized water before it isapplied to the silicon nanoparticles 12.

After the carbon precursor is separated from the altered mixture andwashed, the carbon precursor may be dried at a temperature ranging fromabout 60° C. to about 100° C. for a time period ranging from about 6hours to about 24 hours. The drying of the precursor may also be undervacuum.

In one specific example, the carbon precursor is synthesized byrefluxing, with vigorously stirring, 1.72 g of9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester, 1.0 g2,7-dibromofluorene, 20 mg (PPh₃)₄Pd(0), 50 g THF, and 5 mL of a 2 MNa₂CO₃ solution for about 72 hours in an inert gas atmosphere. In thisexample, the carbon precursor may then be filtered, washed by water, anddried under vacuum at 60° C. overnight (i.e., for 12 hours). The oxygenatoms from 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)ester are present in a byproduct (Na₂CO₃—BO₂C₃H₆) which is soluble inTHF and therefore, removed during filtering.

After the carbon precursor is applied on the silicon nanoparticles 12,the method includes heating the silicon nanoparticles 12 (with thecarbon precursor thereon) to form the carbon coating 14 on the siliconnanoparticles 12. In addition to forming the carbon coating 14, theheating also causes the carbon coating 14 to develop electricalconductivity. In an example, the silicon nanoparticles 12 (with thecarbon precursor thereon) are heated at a temperature ranging from about650° C. to about 750° C. for a time period ranging from about 1 hour toabout 10 hours. In another example, the silicon nanoparticles 12 (withthe carbon precursor thereon) are heated at a temperature of 720° C. fora time period ranging from about 1 hour to about 10 hours.

The heating of the silicon nanoparticles 12 may be performed in an inertgas atmosphere. In some examples of the method, the siliconnanoparticles 12 may be purged with an inert gas prior to being heatedto form the carbon coating 14. In one such example, the siliconnanoparticles 12 with the carbon precursor applied thereon may be purgedwith argon gas after being placed in a closed system (e.g., a furnace)and prior to being heated. While not being bound to any theory, it isbelieved that heating in an inert gas atmosphere and/or purging thesilicon nanoparticles 12 with an inert gas may help the structure of thecarbon coating 14 to remain oxygen-free at a heating temperature rangingfrom about 650° C. to about 750° C. The oxygen-free structure of thecarbon coating 14 allows the carbon coating 14 to mitigate the highvolume expansion, low electrical conductivity, poor cycle life, andactive surface SEI formation of silicon electrodes, without consumingactive lithium and electrolyte or forming an electrically insulatingsilicon carbide layer.

As shown in FIG. 1 , the structure of the carbon coating 14, in additionto being oxygen-free, includes pentagon rings 16. In some examples, thestructure of the carbon coating 14 further includes heptagon rings 18.The presence of the pentagon rings 16 and/or the heptagon rings 18 inthe structure of the carbon coating 14 may give rise to curvatures 20 inthe carbon coating 14. While not being bound to any theory, it isbelieved that the presence of these curvatures 20 in the carbon coating14 may help the coating 14 resist the volume expansion of silicon.

As shown in FIG. 1 , the carbon coating 14 may have pores 22. When thesilicon nanoparticles 12 are heated to form the carbon coating 14, thecarbon precursor may release small molecule gases (e.g., hydrogen gas,water vapor, and carbon dioxide). These small molecule gases form pores22 in the carbon coating 14. When the carbon coated siliconnanoparticles 10 are incorporated into the negative electrode 24, 24′(see FIGS. 2 and 3 ) of a battery, the electrolyte may fill the pores 22and allow lithium ions to be conducted to and from the siliconnanoparticles 12.

As mentioned above, the presence of allyl groups in the oxygen-free,fluorene-based polymer may affect the porosity of the carbon coating 14.Allyl groups may promote the crosslinking of the oxygen-free,fluorene-based polymer, and crosslinking may prevent the polymer fromsoftening or melting during the heating of the silicon nanoparticles 12.Polymer melting or softening may cause the carbon coating 14 to beinsufficiently porous. If the carbon coating 14 is insufficientlyporous, too few lithium ions will be conducted to and from the siliconnanoparticles 12.

The pores 22 of the carbon coating 14 may be any shape (e.g., circular,elongated, or irregularly shaped). The pores 22 of the carbon coating 14may also be any size. In an example, 80% of the pores 22 in the carboncoating 14 are meso-sized (i.e., from about 2 nm to about 50 nm indiameter). In another example, the average diameter of the pores 22ranges from 2 nm to 50 nm. In still another example, the pore volume mayrange from about 0.1 cm³/g to about 0.5 cm³/g.

The carbon coating 14 may also have a surface area ranging from about 50m²/g to about 300 m²/g. If the surface area of the carbon coating 14 istoo high (e.g., greater than 1,000 m²/g), a large amount of activelithium may be consumed to form SEI. The surface area of the carboncoating 14 may be affected by the number of pores 22 in the coating 14,the size of the pores 22 in the coating 14, and the thickness of thecoating 14. In an example, the carbon coating 14 may have a thicknessranging from about 1 nm to about 10 nm.

After obtaining the coated nanoparticles 10 (i.e., silicon nanoparticles12 having the carbon coating 14 thereon), the carbon coated siliconnanoparticle 10 may be added, as an active material, to a negativeelectrode composition to form a negative electrode 24, 24′ for use in alithium-based battery. An example of the method for preparing a negativeelectrode 24, 24′ of a lithium-based battery 400, 500 (see FIGS. 4 and 5) will now be discussed in reference to FIGS. 2 and 3 . FIG. 2 depictsan example of a negative electrode 24 including the carbon coatedsilicon nanoparticles 10 as an active material, a binder 26, and aconductive filler 28, on a support 30. FIG. 3 depicts an example of anegative electrode 24′ including the carbon coated silicon nanoparticles10 as an active material, a binder 26, a conductive filler 28, and anadditional active material 32, on a support 30.

In examples of preparing the negative electrode 24, the carbon coatedsilicon nanoparticles 10 are dry mixed with the conductive filler 28. Inexamples of preparing the negative electrode 24′, the carbon coatedsilicon nanoparticles 10 are dry mixed with the additional activematerial 32 and the conductive filler 28. In some instances, the binder26 is also dry mixed with the other components 10, 28 or 10, 28, 32. Asolvent may then be added to the dry mixture. In other instances, thebinder 26 and solvent are mixed together, and then added to the drymixed components 10, 28 or 10, 28, 32. As will be discussed in moredetail below, the solvent may be deionized water or an organic solvent,depending on the binder 26 selected to form a dispersion or mixture.

The additional active material 32, included in the negative electrode24′, may be any lithium host active material that may be incorporatedinto the negative electrode 24′ using a slurry coating method and thatcan sufficiently undergo lithium intercalation and deintercalation, orlithium alloying and dealloying, or lithium insertion and deinsertion,while copper or another current collector 30 functions as the negativeterminal of the electrochemical cell/battery. Examples of the lithiumhost active material include graphite or silicon-based materials.Further examples include tin, alloys of tin, antimony, and alloys ofantimony. Graphite exhibits favorable lithium intercalation anddeintercalation characteristics, is relatively non-reactive, and canstore lithium in quantities that produce a relatively high energydensity. Commercial forms of graphite that may be used to fabricate theadditional active material 32 of the negative electrode 24′ areavailable from, for example, Timcal Graphite & Carbon (Bodio,Switzerland), Lonza Group (Basel, Switzerland), or Superior Graphite(Chicago, Ill.). Examples of the silicon-based active material includecrystalline silicon, amorphous silicon, silicon oxide (SiOx), siliconalloys (e.g., Si—Sn), etc. The silicon active material may be in theform of a powder, particles, etc. ranging from nano-size to micro-size.

The carbon coated silicon nanoparticles 10, alone or in combination withthe additional active material 32 may be intermingled with the binder 26and the conductive filler 28. The binder 26 may be used to structurallyhold the carbon coated silicon nanoparticles 10, the conductive filler28, and/or the additional active material 32 together. Some examples ofsuitable binders 26 include polyvinylidene fluoride (PVdF), polyethyleneoxide (PEO), an ethylene propylene diene monomer (EPDM) rubber,carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR),styrene-butadiene rubber carboxymethyl cellulose (SBR-CMC), polyacrylicacid (PAA), cross-linked polyacrylic acid-polyethylenimine, polyimide,or any other suitable binder material. Examples of the still othersuitable binders 26 include polyvinyl alcohol (PVA), sodium alginate, orother water-soluble binders.

The conductive filler 28 may be a conductive carbon material. Theconductive carbon may be a high surface area carbon, such as acetyleneblack or another carbon material (e.g., Super P). Other examples ofsuitable conductive fillers 28 include graphene, graphite, carbonnanotubes, and/or carbon nanofibers. The conductive filler 28 ensureselectron conduction between the negative-side current collector 30 andthe carbon coated silicon nanoparticles 10 and/or the additional activematerial 32.

In an example of the method for making the negative electrode 24, thecarbon coated silicon nanoparticles 10 are mixed with the binder 26 andthe conductive filler 28. In an example of the method for making thenegative electrode 24′, the carbon coated silicon nanoparticles 10 aremixed with the binder 26, the conductive filler 28, and the additionalactive material 32. In either of these examples, all of the componentsmay be manually mixed by dry-grinding. After all the components areground together, the ground components are combined with water ororganic solvent (depending on the binder 26 used) to form thedispersion/mixture. In an example, the solvent is a polar aproticsolvent. Examples of suitable polar aprotic solvents includedimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP),dimethylformamide (DMF), dimethylsulfoxide (DMSO), or another Lewisbase, or combinations thereof. When a water soluble binder, such assodium alginate, is used, the solvent may be water.

The dispersion/mixture may be mixed by milling. Milling aids intransforming the dispersion/mixture into a coatable slurry. Low-shearmilling or high-shear milling may be used to mix the dispersion/mixture.The dispersion/mixture milling time ranges from about 10 minutes toabout 20 hours depending on the milling shear rate. In an example, arotator mixer is used for about 20 minutes at about 2000 rpm to mill thedispersion/mixture.

The carbon coated silicon nanoparticles 10 may be present in thedispersion/mixture in an amount ranging from about 5 wt % to about 95 wt% (based on total solid wt % of the dispersion/mixture). When theadditional active material 32 is not present in the dispersion/mixture,the carbon coated silicon nanoparticles 10 may be present in a greateramount (e.g., an amount ranging from about 50 wt % to about 95 wt %) asthe coated nanoparticles function as the sole active material. When theadditional active material 32 is present in the dispersion/mixture, thecarbon coated silicon nanoparticles 10 may be present in a lesser amount(e.g., an amount ranging from about 5 wt % to about 45 wt %). When it ispresent in the dispersion/mixture, the additional active material 32 maybe present in an amount ranging from about 50 wt % to about 95 wt %(based on total solid wt % of the dispersion/mixture). The binder 26 maybe present in the dispersion/mixture in an amount ranging from about 1wt % to about 20 wt % (based on total solid wt % of thedispersion/mixture), and the conductive filler 28 may be present in thedispersion/mixture in an amount ranging from about 1 wt % to about 20 wt% (based on total solid wt % of the dispersion/mixture).

The slurry is then deposited onto the support 30. In an example, thesupport 30 is the negative-side current collector. In another example,the support 30 is a foil support. It is to be understood that thesupport 30 may be formed from copper, nickel, or any other appropriateelectrically conductive material known to skilled artisans. The support30 that is selected should be capable of collecting and moving freeelectrons to and from an external circuit connected thereto.

The slurry may be deposited using any suitable technique. As examples,the slurry may be cast on the surface of the support 30, or may bespread on the surface of the support 30, or may be coated on the surfaceof the support 30 using a slot die coater.

The deposited slurry may be exposed to a drying process in order toremove any remaining solvent and/or water. Drying may be accomplishedusing any suitable technique. For example, the drying is conducted atambient conditions (i.e., at room temperature, about 18° C. to 22° C.,and 1 atmosphere). Drying may be performed at an elevated temperatureranging from about 60° C. to about 150° C. In some examples, vacuum mayalso be used to accelerate the drying process. As one example of thedrying process, the deposited slurry may be exposed to vacuum at about100° C. for about 12 to 24 hours.

The drying process results in the formation of the negative electrode24, 24′. In an example, the thickness of the dried slurry (i.e.,negative electrode 24, 24′) ranges from about 5 μm to about 200 μm. Inanother example, the thickness of the dried slurry (i.e., negativeelectrode 24, 24′) ranges from about 10 μm to about 100 μm.

During the formation of the negative electrode 24, 24′, the water and/ororganic solvent(s) is/are removed, and thus the resulting negativeelectrode 24, 24′ may include the carbon coated silicon nanoparticles10, the binder 26, the conductive filler 28, and/or the additionalactive material 32 in the following amounts. The carbon coated siliconnanoparticles 10 may be present in the negative electrode 24, 24′ in anamount ranging from about 5 wt % to about 95 wt % (based on the total wt% of the negative electrode 24, 24′). When the additional activematerial 32 is not present in the negative electrode 24, the carboncoated silicon nanoparticles 10 may be present in a greater amount(e.g., an amount ranging from about 50 wt % to about 95 wt %), and whenthe additional active material 32 is present in the negative electrode24′, the carbon coated silicon nanoparticles 10 may be present in alesser amount (e.g., an amount ranging from about 5 wt % to about 45 wt%). When it is present in the negative electrode 24′, the additionalactive material 32 may be present in an amount ranging from about 50 wt% to about 95 wt % (based on the total wt % of the negative electrode24′). The binder 26 may be present in the negative electrode 24, 24′ inan amount ranging from about 1 wt % to about 20 wt % (based on the totalwt % of the negative electrode 24, 24′), and the conductive filler 28may be present in the negative electrode 24, 24′ in an amount rangingfrom about 1 wt % to about 20 wt % (based on the total wt % of thenegative electrode 24, 24′).

If the negative electrode 24, 24′ is to be paired with a positiveelectrode that is not formed of lithium, the negative electrode 24, 24′may be exposed to a pre-lithiation process prior to incorporating itinto the electrochemical cell/battery. The pre-lithiation techniquelithiates the negative electrode 24, 24′. In an example, the negativeelectrode 24, 24′ may then be pre-lithiated by applying lithium powder(such as stabilized lithium metal powder (SLMP) from FMC Company) orlithium foil onto the electrode, followed by calendaring theelectrode/Lithium powder (or foil).

The pre-lithiated negative electrode 24, 24′ may then be used in anelectrochemical cell/battery. In general, the cell/battery may beassembled with the negative electrode 24, 24′, a suitable positiveelectrode (examples of which will be described below), a microporouspolymer separator positioned between the negative and positiveelectrodes, and an example of the electrolyte disclosed herein includinga suitable solvent for the particular battery type.

Sulfur Battery/Electrochemical Cell

An example of a sulfur battery 400 is shown in FIG. 4 . For the sulfurbattery/electrochemical cell 400, any example of the negative electrode24, 24′ (e.g., with or without the additional active material 32) may beused.

The positive electrode 34 of the sulfur battery 400 may include anysulfur-based active material that can sufficiently undergo lithiumalloying and dealloying with aluminum or another suitable currentcollector 35 functioning as the positive terminal of the sulfurelectrochemical cell. An example of the sulfur-based active material isa sulfur-carbon composite. In an example, the weight ratio of S to C inthe positive electrode 34 ranges from 1:9 to 9:1. The positive electrode34 in the sulfur battery 400 may include any of the previously mentionedbinders 26 and conductive fillers 28.

The microporous polymer separator 36 may be formed, e.g., from apolyolefin. The polyolefin may be a homopolymer (derived from a singlemonomer constituent) or a heteropolymer (derived from more than onemonomer constituent), and may be either linear or branched. If aheteropolymer derived from two monomer constituents is employed, thepolyolefin may assume any copolymer chain arrangement including those ofa block copolymer or a random copolymer. The same holds true if thepolyolefin is a heteropolymer derived from more than two monomerconstituents. As examples, the polyolefin may be polyethylene (PE),polypropylene (PP), a blend of PE and PP, or multi-layered structuredporous films of PE and/or PP. Commercially available porous separators36 include single layer polypropylene membranes, such as CELGARD 2400and CELGARD 2500 from Celgard, LLC (Charlotte, N.C.). It is to beunderstood that the microporous separator 36 may be coated or treated,or uncoated or untreated. For example, the microporous separator 36 mayor may not be coated or include any surfactant treatment thereon.

In other examples, the microporous separator 36 may be formed fromanother polymer chosen from polyethylene terephthalate (PET),polyvinylidene fluoride (PVdF), polyamides (Nylons), polyurethanes,polycarbonates, polyesters, polyetheretherketones (PEEK),polyethersulfones (PES), polyimides (PI), polyamide-imides, polyethers,polyoxymethylene (e.g., acetal), polybutylene terephthalate,polyethylenenaphthenate, polybutene, polyolefin copolymers,acrylonitrile-butadiene styrene copolymers (ABS), polystyrenecopolymers, polymethylmethacrylate (PMMA), polyvinyl chloride (PVC),polysiloxane polymers (such as polydimethylsiloxane (PDMS)),polybenzimidazole (PBI), polybenzoxazole (PBO), polyphenylenes (e.g.,PARMAX™ (Mississippi Polymer Technologies, Inc., Bay Saint Louis,Miss.)), polyarylene ether ketones, polyperfluorocyclobutanes,polytetrafluoroethylene (PTFE), polyvinylidene fluoride copolymers andterpolymers, polyvinylidene chloride, polyvinylfluoride, liquidcrystalline polymers (e.g., VECTRAN™ (Hoechst AG, Germany) and ZENITE®(DuPont, Wilmington, Del.)), polyaramides, polyphenylene oxide, and/orcombinations thereof. It is believed that another example of a liquidcrystalline polymer that may be used for the microporous separator 36 ispoly(p-hydroxybenzoic acid). In yet another example, the microporousseparator 36 may be chosen from a combination of the polyolefin (such asPE and/or PP) and one or more of the other polymers listed above.

The microporous separator 36 may be a single layer or may be amulti-layer (e.g., bilayer, trilayer, etc.) laminate fabricated fromeither a dry or wet process. For example, a single layer of thepolyolefin and/or other listed polymer may constitute the entirety ofthe microporous polymer separator 36. As another example, however,multiple discrete layers of similar or dissimilar polyolefins and/orpolymers may be assembled into the microporous polymer separator 36. Inone example, a discrete layer of one or more of the polymers may becoated on a discrete layer of the polyolefin to form the microporouspolymer separator 36. Further, the polyolefin (and/or other polymer)layer, and any other optional polymer layers, may further be included inthe microporous polymer separator 36 as a fibrous layer to help providethe microporous polymer separator 36 with appropriate structural andporosity characteristics. Still other suitable microporous polymerseparators 36 include those that have a ceramic layer attached thereto,and those that have ceramic filler in the polymer matrix (i.e., anorganic-inorganic composite matrix).

The microporous separator 36 operates as both an electrical insulatorand a mechanical support, and is sandwiched between the negativeelectrode 24, 24′ and the positive electrode 34 to prevent physicalcontact between the two electrodes 24, 34 or 24′, 34 and the occurrenceof a short circuit. In addition to providing a physical barrier betweenthe electrodes 24, 34 or 24′, 34, the microporous polymer separator 36ensures passage of lithium ions through the electrolyte filling itspores.

The pre-lithiated negative electrode 24, 24′ (e.g., including Si, SiOx,graphite, Sn or combinations thereof), the sulfur based positiveelectrode 34, and the microporous separator 36 are soaked with theelectrolyte (not shown), including a solvent suitable for the sulfurbattery 400 and a lithium salt.

In an example, the solvent suitable for the lithium sulfur battery 400may be an ether based solvent. Examples of the ether based solventinclude cyclic ethers, such as 1,3-dioxolane, tetrahydrofuran,2-methyltetrahydrofuran, and chain structure ethers, such as1,2-dimethoxyethane, 1-2-diethoxyethane, ethoxymethoxyethane,tetraethylene glycol dimethyl ether (TEGDME), polyethylene glycoldimethyl ether (PEGDME), ethyl ether, aliphatic ethers, polyethers, andmixtures thereof.

Examples of the lithium salt that may be dissolved in the ether(s)include LiPF₆, LiClO₄, LiAlCl₄, LiI, LiBr, LiSCN, LiBF₄, LiB(C₆H₅)₄,LiAsF₆, LiCF₃SO₃, LiN(FSO₂)₂(LIFSI), LiN(CF₃SO₂)₂(LITFSI or lithiumbis(trifluoromethylsulfonyl)imide), LiB(C₂O₄)₂ (LiBOB), LiBF₂(C₂O₄)(LiODFB), LiPF₃(C₂F₅)₃ (LiFAP), LiPF₄(CF₃)₂, LiPF₄(C₂O₄) (LiFOP),LiPF₃(CF₃)₃, LiSO₃CF₃, LiNO₃, and mixtures thereof.

Lithium Ion Battery/Electrochemical Cell

An example of a lithium ion battery 500 is shown in FIG. 5 . For thelithium ion battery/electrochemical cell 500, any example of thenegative electrode 24, 24′ (e.g., with or without the additional activematerial 32) may be used.

The positive electrode 34′ of the lithium ion battery 500 may includeany lithium-based or non-lithium-based active material that cansufficiently undergo lithium insertion and deinsertion with aluminum oranother suitable current collector 35 functioning as the positiveterminal of the lithium ion electrochemical cell.

One common class of known lithium-based active materials suitable forthis example of the positive electrode 34′ includes layered lithiumtransition metal oxides. For example, the lithium-based active materialmay be spinel lithium manganese oxide (LiMn₂O₄), lithium cobalt oxide(LiCoO₂), a manganese-nickel oxide spinel [Li(Mn_(1.5)Ni_(0.5))O₂], or alayered nickel-manganese-cobalt oxide (having a general formula ofxLi₂MnO₃·(1−x)LiMO₂ or (M is composed of any ratio of Ni, Mn and/or Co).A specific example of the layered nickel-manganese-cobalt oxide includes(xLi₂MnO₃·(1−x)Li(Ni_(1/3)Mn_(1/3)Co_(1/3))O₂). Other suitablelithium-based active materials include Li(Ni_(1/3)Mn_(1/3)Co_(1/3))O₂,Li_(x+y)Mn_(2−y)O₄ (LMO, 0<x<1 and 0<y<0.1), or a lithium iron polyanionoxide, such as lithium iron phosphate (LiFePO₄) or lithium ironfluorophosphate (Li₂FePO₄F), or a lithium rich layer-structure. Stillother lithium-based active materials may also be utilized, such asLiNi_(1−x)Co_(1−y)M_(x+y)O₂ or LiMn_(1.5−x)Ni_(0.5−y)M_(x+y)O₄ (M iscomposed of any ratio of Al, Ti, Cr, and/or Mg), stabilized lithiummanganese oxide spinel (Li_(x)Mn_(2−y)M_(y)O₄, where M is composed ofany ratio of Al, Ti, Cr, and/or Mg), lithium nickel cobalt aluminumoxide (e.g., LiNi_(0.8)Co_(0.15)Al_(0.05)O₂) or NCA), aluminumstabilized lithium manganese oxide spinel (e.g.,Li_(x)Al_(0.05)Mn_(0.95)O₂), lithium vanadium oxide (LiV₂O₅), Li₂MSiO₄(where M is composed of any ratio of Co, Fe, and/or Mn), and any otherhigh energy nickel-manganese-cobalt material (HE-NMC, NMC orLiNiMnCoO₂). By “any ratio” it is meant that any element may be presentin any amount. So, in some examples, M could be Al, with or without Cr,Ti, and/or Mg, or any other combination of the listed elements. Inanother example, anion substitutions may be made in the lattice of anyexample of the lithium transition metal based active material tostabilize the crystal structure. For example, any 0 atom may besubstituted with an F atom.

Suitable non-lithium based materials for this example of the positiveelectrode 34′ include metal oxides, such as manganese oxide (Mn₂O₄),cobalt oxide (CoO₂), a nickel-manganese oxide spinel, a layerednickel-manganese-cobalt oxide, or an iron polyanion oxide, such as ironphosphate (FePO₄) or iron fluorophosphate (FePO₄F), or vanadium oxide(V₂O₅).

The positive electrode 34′ in the lithium ion electrochemicalcell/battery 500 may include any of the previously mentioned binders 26and conductive fillers 28.

The lithium ion electrochemical cell/battery 500 may also include any ofthe previously provided examples of the microporous polymer separator36.

The negative electrode 24, 24′, the positive electrode 34′, and themicroporous separator 36 are soaked with the electrolyte (not shown),including a solvent suitable for the lithium ion battery 500 and alithium salt.

In an example, the solvent suitable for the lithium ion battery 500 maybe an organic solvent or a mixture of organic solvents. Examples ofsuitable organic solvents include cyclic carbonates (ethylene carbonate,propylene carbonate, butylene carbonate, fluoroethylene carbonate),linear carbonates (dimethyl carbonate (DMC), diethyl carbonate,ethylmethyl carbonate), aliphatic carboxylic esters (methyl formate,methyl acetate, methyl propionate), γ-lactones (γ-butyrolactone,γ-valerolactone), chain structure ethers (1,2-dimethoxyethane,1-2-diethoxyethane, ethoxymethoxyethane, tetraglyme), cyclic ethers(tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane), dioxane,acetonitrile, nitromethane, ethyl monoglyme, phosphoric triesters,trimethoxymethane, dioxolane derivatives, 3-methyl-2-oxazolidinone,propylene carbonate derivatives, tetrahydrofuran derivatives, ethylether, 1,3-propanesultone, N-methyl acetamide, acetals, ketals,sulfones, sulfolanes, aliphatic ethers, cyclic ethers, glymes,polyethers, phosphate esters, siloxanes, dioxolanes,N-alkylpyrrolidones, and mixtures thereof.

Examples of the lithium salt that may be dissolved in the organicsolvent(s) include all of the lithium salts listed above that may bedissolved in the ether(s).

As shown in FIGS. 4 and 5 , the lithium sulfur battery/electrochemicalcell 400 and lithium ion battery/electrochemical cell 500 each includean interruptible external circuit 38 that connects the negativeelectrode 24, 24′ and the positive electrode 34, 34′. The lithium sulfurbattery/electrochemical cell 400 and lithium ion battery/electrochemicalcell 500 each may also support a load device 40 that can be operativelyconnected to the external circuit 38. The load device 40 receives a feedof electrical energy from the electric current passing through theexternal circuit 38 when the battery 400, 500 is discharging. While theload device 40 may be any number of known electrically-powered devices,a few specific examples of a power-consuming load device 40 include anelectric motor for a hybrid vehicle or an all-electrical vehicle, alaptop computer, a cellular phone, and a cordless power tool. The loaddevice 40 may also, however, be an electrical power-generating apparatusthat charges the battery 400, 500 for purposes of storing energy. Forinstance, the tendency of windmills and solar panels to variably and/orintermittently generate electricity often results in a need to storesurplus energy for later use.

FIGS. 4 and 5 also illustrate the porous separator 36 positioned betweenthe electrodes 24, 24′, 34, 34′. Metal contacts/supports (e.g., a copperfoil contact/support or a nickel foil contact/support) may be made tothe electrodes 24, 24′, 34, 34′, examples of which include anegative-side current collector 30 to the negative electrode 24, 24′,and a positive-side current collector 35 to the positive electrode 34,34′.

The lithium sulfur battery/electrochemical cell 400 and/or lithium ionbattery/electrochemical cell 500 may also include a wide range of othercomponents that, while not depicted here, are nonetheless known toskilled artisans. For instance, the battery 400, 500 may include acasing, gaskets, terminals, tabs, and any other desirable components ormaterials that may be situated between or around the negative electrode24, 24′ and the positive electrode 34, 34′ for performance-related orother practical purposes. Moreover, the size and shape of the battery400, 500, as well as the design and chemical make-up of its maincomponents, may vary depending on the particular application for whichit is designed. Battery-powered automobiles and hand-held consumerelectronic devices, for example, are two instances where the battery400, 500 would most likely be designed to different size, capacity, andpower-output specifications. The battery 400, 500 may also be connectedin series and/or in parallel with other similar batteries to produce agreater voltage output and current (if arranged in parallel) or voltage(if arranged in series) if the load device 40 so requires.

The lithium sulfur battery/electrochemical cell 400 and lithium ionbattery/electrochemical cell 500 each generally operates by reversiblypassing lithium ions between the negative electrode 24, 24′ and thepositive electrode 34, 34′. In the fully charged state, the voltage ofthe battery 400, 500 is at a maximum (typically in the range 2.0V to5.0V); while in the fully discharged state, the voltage of the battery400, 500 is at a minimum (typically in the range 0V to 2.0V).Essentially, the Fermi energy levels of the active materials in thepositive and negative electrodes 34, 34′, 24, 24′ change during batteryoperation, and so does the difference between the two, known as thebattery voltage. The battery voltage decreases during discharge, withthe Fermi levels getting closer to each other. During charge, thereverse process is occurring, with the battery voltage increasing as theFermi levels are being driven apart. During battery discharge, theexternal load device 40 enables an electronic current flow in theexternal circuit 38 with a direction such that the difference betweenthe Fermi levels (and, correspondingly, the cell voltage) decreases. Thereverse happens during battery charging: the battery charger forces anelectronic current flow in the external circuit 38 with a direction suchthat the difference between the Fermi levels (and, correspondingly, thecell voltage) increases.

At the beginning of a discharge, the negative electrode 24, 24′ of thebattery 400, 500 contains a high concentration of inserted (e.g.,alloyed, intercalated, etc.) lithium while the positive electrode 34,34′ is relatively depleted. When the negative electrode 24, 24′ containsa sufficiently higher relative quantity of inserted lithium, thelithium-based battery 400, 500 can generate a beneficial electriccurrent by way of reversible electrochemical reactions that occur whenthe external circuit 38 is closed to connect the negative electrode 24,24′ and the positive electrode 34, 34′. The establishment of the closedexternal circuit 38 under such circumstances causes the extraction ofinserted lithium from the negative electrode 24, 24′. The extractedlithium atoms are split into lithium ions and electrons as they leave ahost (i.e., the active material(s)) at the negativeelectrode-electrolyte interface.

The chemical potential difference between the positive electrode 34, 34′and the negative electrode 24, 24′ (ranging from about 0.005V to about5.0V, depending on the exact chemical make-up of the electrodes 24, 24′,34, 34′) drives the electrons produced by the oxidation of insertedlithium at the negative electrode 24, 24′ through the external circuit38 towards the positive electrode 34, 34′. The lithium ions areconcurrently carried by the electrolyte solution through the microporousseparator 36 towards the positive electrode 34, 34′. The electronsflowing through the external circuit 38 and the lithium ions migratingacross the microporous separator 36 in the electrolyte solutioneventually incorporate, in some form, lithium at the positive electrode34, 34′. The electric current passing through the external circuit 38can be harnessed and directed through the load device 40 until the levelof inserted lithium in the negative electrode 24, 24′ falls below aworkable level or the need for electrical energy ceases.

The battery 400, 500 may be recharged after a partial or full dischargeof its available capacity. To charge the battery 400, 500 an externalbattery charger is connected to the positive and the negative electrodes24, 24′, 34, 34′ to drive the reverse of battery dischargeelectrochemical reactions. During recharging, the electrons flow backtowards the negative electrode 24, 24′ through the external circuit 38,and the lithium ions are carried by the electrolyte across themicroporous separator 36 back towards the negative electrode 24, 24′.The electrons and the lithium ions are reunited at the negativeelectrode 24, 24′, thus replenishing it with inserted lithium forconsumption during the next battery discharge cycle.

The external battery charger that may be used to charge the battery 400,500 may vary depending on the size, construction, and particular end-useof the battery 400, 500. Some suitable external battery chargers includea battery charger plugged into an AC wall outlet and a motor vehiclealternator.

The battery 400, 500 including the negative electrode 24, 24′, whichincludes the carbon coated silicon nanoparticles 10, may have a firstcycle efficiency greater than about 70%. The battery 400, 500 may alsohave a stable cycling performance for many cycles. In one example, thebattery 400, 500 has a loading of the carbon coated siliconnanoparticles 10 of about 1.5 mg/cm², and a stable cycling performancefor about 300 cycles. In another example, the battery 400, 500 has aloading of the carbon coated silicon nanoparticles 10 of about 0.5mg/cm², and a stable cycling performance for about 1,000 cycles.

To further illustrate the present disclosure, an example is givenherein. It is to be understood that this example is provided forillustrative purposes and is not to be construed as limiting the scopeof the disclosure.

EXAMPLE

Two example negative electrodes and two comparative negative electrodeswere prepared. Example carbon coated silicon nanoparticles were preparedaccording to an example of the method disclosed herein, and were used asthe active material in the example negative electrode. Comparativecarbon coated silicon nanoparticles were purchased and were used as theactive material in the comparative negative electrodes.

The oxygen-free, fluorene-base polymer was used as the carbon precursorto form the example carbon coated silicon nanoparticles. Theoxygen-free, fluorene-base polymer was synthesized by vigorouslystirring 1.72 g of 9,9-dioctylfluorene-2,7-diboronic acidbis(1,3-propanediol) ester, 1.0 g 2,7-dibromofluorene, 20 mgtetrakis(triphenylphosphine)palladium(0), 50 g tetrahydrofuran, and 5 mLof a 2 M sodium carbonate solution for about 72 hours in an argon gasatmosphere. Then, the synthesized polymer was filtered, washed withwater, and dried overnight (i.e., for 12 hours) under vacuum at 60° C.

Then, 1 g of the synthesized polymer and 5 g of silicon nanoparticleswere mixed in 10 g of chlorobenzene by a THINKY® Mixer to form a slurry.The slurry was then dried in a hood.

The dried mixture was then transferred to a furnace and purged withargon gas. The purged mixture was heated by the furnace to 720° C. at arate of 10° C./minute and was held at 720° C. for 2 hours to form thecarbon coated silicon nanoparticles. After cool down, the mixture waspestle pressed to make fine powders.

The carbon coating of the comparative carbon coated siliconnanoparticles had been formed from a resorcinal-formaldehyde (RF)polymer, which includes oxygen.

The example carbon coated silicon nanoparticles were incorporated as anactive martial in example negative electrodes. The comparative, RFcarbon coated silicon nanoparticles were incorporated as activematerials in comparative negative electrodes. An additional activematerial was not used in the example negative electrodes or in thecomparative negative electrodes. Each of the example negative electrodesincluded about 70 wt % of the example carbon coated siliconnanoparticles, about 15 wt % of a binder (sodium alginate), and about 15wt % of a conductive filler (carbon black). Each of the comparativenegative electrodes included all of the same components as the examplenegative electrode except for the type of carbon coated siliconnanoparticles. As mentioned above, the comparative negative electrodesincluded carbon coated silicon nanoparticles formed using aresorcinal-formaldehyde polymer as the carbon precursor.

To form each of the example and comparative negative electrodes, therespective carbon coated silicon nanoparticles and the carbon black weredry mixed in a THINKY® mixer. The sodium alginate, carbon black and asolvent (water) were added to each of the dry mixtures. The mixtureswere mixed until relatively uniform, coatable slurries were formed. Theslurries were casted onto respective copper current collectors. Theexample electrode coating and comparative electrode coating were driedat room temperature in air, then dried in an oven at about 80° C. forabout 24 hours, and then dried in vacuum at about 100° C. for about 24hours.

Each of the example and comparative negative electrodes was used in ahalf cell. Microporous tri-layered polypropylene (PP) and polyethylene(PE) polymer membranes (CELGARD 2032, available from Celgard) were usedas the separators. Two types electrolyte for the RF coated Si—Li celland the oxygen-free, fluorene-base polymer coated Si—Li cell have beenused: for comparative example 3 and example 4: 1M LiPF₆-DMC:FEC withdimethyl carbonate (DMC) and fluoroethylene carbonate (FEC) at a volumeratio of 4:1 or for comparative example 1 and example 2: 1MLiPF₆-EC:EMC+10% FEC with ethylene carbonate (EC) and ethylmethylcarbonate (EMC) at a volume ratio of 1:2 and 10% fluoroethylenecarbonate (FEC).

The test conditions for the example and comparative cells were: roomtemperature; current=500 μA; and voltage window ranging from 50 mV to1.0 V. The capacity retention results are shown in FIG. 6 . In FIG. 6 ,the left Y axis, labeled “C,” represents the capacity retention (inmAh), and the X axis, labeled “#,” represents the cycle number.

As illustrated in FIG. 6 , throughout the cycles, the capacity retentionof the example cells (labeled “2” and “4”) was generally higher than thecapacity retention of the respective comparative cells (labeled “1” and“3”) that were formed with the same electrolyte. Comparative example 1and example 2 were prepared using an inferior electrolyte (i.e., 1MLiPF₆-EC:EMC+10% FEC), and the difference in performance between thecomparative example and the example is more evident with the improvedelectrolyte (i.e., 1M LiPF₆-DMC:FEC). Clearly, the performance ofcomparative example 3 was not as good as the performance of example 4,both of which were formed with the improved electrolyte. However, it isnoted that example 2 did perform better than comparative example 1 for75 cycles, even with the inferior electrolyte.

Further, the example cell 4 showed improved first cycle efficiency overthe first cycle efficiency of the comparative example cell 3 (RF coatedSi). The first cycle efficiency of the example cell 4 (oxygen-free,fluorene-base polymer coated Si) was 75%. The first cycle efficiency ofthird comparative cell 3 was 69%.

Reference throughout the specification to “one example”, “anotherexample”, “an example”, and so forth, means that a particular element(e.g., feature, structure, and/or characteristic) described inconnection with the example is included in at least one exampledescribed herein, and may or may not be present in other examples. Inaddition, it is to be understood that the described elements for anyexample may be combined in any suitable manner in the various examplesunless the context clearly dictates otherwise.

It is to be understood that the ranges provided herein include thestated range and any value or sub-range within the stated range. Forexample, a range from about 650° C. to about 750° C. should beinterpreted to include not only the explicitly recited limits of fromabout 650° C. to about 750° C., but also to include individual values,such as 675° C., 685.5° C., 725° C., etc., and sub-ranges, such as fromabout 700.25° C. to about 728° C., etc. Furthermore, when “about” isutilized to describe a value, this is meant to encompass minorvariations (up to +/−10%) from the stated value.

In describing and claiming the examples disclosed herein, the singularforms “a”, “an”, and “the” include plural referents unless the contextclearly dictates otherwise.

While several examples have been described in detail, it is to beunderstood that the disclosed examples may be modified. Therefore, theforegoing description is to be considered non-limiting.

What is claimed is:
 1. A method, comprising: providing siliconnanoparticles; applying a carbon precursor on the silicon nanoparticles,the carbon precursor being an oxygen-free, fluorene-based polymer; andthen heating the silicon nanoparticles in an inert gas atmosphere,thereby forming a carbon coating on the silicon nanoparticles, thecarbon coating having an oxygen-free structure including pentagon rings.2. The method as defined in claim 1, wherein the oxygen-free,fluorene-based polymer includes an allyl group.
 3. The method as definedin claim 1, the providing comprises providing silicon nanoparticleshaving a particle size ranging from about 30 nm to about 100 nm.
 4. Themethod as defined in claim 1, wherein the applying of the carbonprecursor on the silicon nanoparticles is accomplished by, mixing thecarbon precursor, the silicon nanoparticles, and a solvent to form aslurry, and drying the slurry.
 5. The method as defined in claim 4,wherein the slurry includes the carbon precursor in an amount 1 weightpercent to about 10 weight percent and the silicon nanoparticles in anamount ranging from about 10 weight percent to about 50 weight percent.6. The method as defined in claim 1, wherein a weight ratio of thecarbon precursor to the silicon nanoparticles is 1:5.
 7. The method asdefined in claim 1, wherein the heating of the silicon nanoparticles isaccomplished at a temperature ranging from about 650° C. to about 750°C.
 8. The method as defined in claim 1, wherein the heating is performedfor a time period ranging from about 1 hour to about 10 hours.
 9. Themethod as defined in claim 1, further comprising synthesizing the carbonprecursor via condensation reaction of a first fluorene-based monomerand a second fluorene-based monomer.
 10. The method as defined in claim9, wherein the first fluorene-based monomer is selected from a groupconsisting of: (a) 9,9-dioctylfluorene-2,7-diboronic acidbis(1,3-propanediol) ester

(b) 9,9-Dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester

(c) 9,9-Didodecylfluorene-2,7-diboronic acid

 or (d) 9,9-Dihexylfluorene-2,7-diboronic acid

 and the second fluorene-based monomer is selected from a groupconsisting of: (a) 2,7-dibromofluorene

(b) 2,7-Dibromo-9,9-dimethyl-9H-fluorene

(c) 2,7-Dibromo-9,9-diallyl-9H-fluorene

(d) 9,9-Didodecyl-2,7-dibromofluorene

 or (e) 9,9-Dihexyl-2,7-dibromofluorene


11. The method as defined in claim 10, wherein the first fluorene-basedmonomer is the 9,9-dioctylfluorene-2,7-diboronic acidbis(1,3-propanediol) ester

 and the second fluorene-based monomer is the 2,7-dibromofluorene


12. The method as defined in claim 9, wherein a molar ratio of the firstfluorene-based monomer to the second fluorene-based monomer is about1:1.
 13. The method as defined in claim 9, wherein synthesizingcomprises, stirring a mixture of the first fluorene-based monomer, thesecond fluorene-based monomer, a solvent, and a catalyst to form thecarbon precursor, and separating the carbon precursor from a remainderof the mixture.
 14. The method as defined in claim 13, wherein theseparating comprises, vacuum filtration or centrifugal force separation.15. The method as defined in claim 13, wherein the catalyst comprises apalladium-based catalyst.
 16. The method as defined in claim 1, furthercomprising: dry mixing the silicon nanoparticles having the carboncoating thereon into a mixture, the mixture including a conductivefiller; adding a binder and a solvent to the mixture; mixing the mixtureto form a slurry; depositing the slurry onto a support; and drying theslurry.
 17. The method as defined in claim 16, further comprising addingan additional active material to the mixture, wherein the additionalactive material is selected from the group consisting of graphite, tin,alloys of tin, antimony, alloys of antimony, crystalline silicon,amorphous silicon, silicon oxide, and silicon alloys.
 18. The method asdefined in claim 1, wherein the oxygen-free, fluorene-based polymer isformed via a condensation reaction between a first fluorene-basedmonomer and a second fluorene-based monomer.
 19. The method as definedin claim 18, wherein the first fluorene-based monomer comprises boronand the second fluorene-based monomer comprises bromine.
 20. A method,comprising: providing silicon nanoparticles; applying a carbon precursoron the silicon nanoparticles, the carbon precursor being an oxygen-free,fluorene-based polymer comprising an allyl group; and then heating thesilicon nanoparticles in an inert gas atmosphere to form a carboncoating on the silicon nanoparticles, the carbon coating having anoxygen-free structure including pentagon rings.